106 research outputs found

    Water transport among the world ocean basins within the water cycle

    Get PDF
    The global water cycle involves water-mass transport on land, in the atmosphere, in the ocean, and among them. Quantification of such transport, especially its time evolution, is essential to identify the footprints of climate change, and it also helps to constrain and improve climatic models. In the ocean, net water-mass transport among the ocean basins is a key process, but it is currently a poorly estimated parameter. We propose a new methodology that incorporates the time-variable gravity observations from the Gravity Recovery and Climate Experiment (GRACE) satellite (2003–2016) to estimate the change in water content; this new approach also overcomes some fundamental limitations of existing methods. We show that the Pacific and Arctic oceans receive an average of 1916 (95 % confidence interval of [1812, 2021]) Gt per month (∼0.72±0.02 Sv) of excess freshwater from the atmosphere and the continents that is discharged into the Atlantic and Indian oceans, where net evaporation minus precipitation returns the water to complete the cycle. This is in contrast to previous GRACE-based studies, where the notion of a see-saw mass exchange between the Pacific and the Atlantic and Indian oceans has been reported. Seasonal climatology as well as the interannual variability of water-mass transport are also reported.This research has been supported by the Spanish Ministry of Science, Innovation and Universities (grant no. RTI2018-093874-B-100)

    On the suitability of the 4° × 4° GRACE mascon solutions for remote sensing Australian hydrology

    Get PDF
    Hydrological monitoring is essential for meaningful water-management policies and actions, especially where water resources are scarce and/or dwindling, as is the case in Australia. In this paper, we investigate the regional 4° × 4° mascon (mass concentration) GRACE solutions for Australia provided by GSFC (Goddard Space Flight Center, NASA) for their suitability in monitoring Australian hydrology, with a particular focus on the Murray-Darling Basin (MDB). Using principal component analysis (PCA) and multi-linear regression analysis (MLRA), the main components of spatial and temporal variability in the mascon solutions are analysed over the whole Australian continent and the MDB. The results are compared to those from global solutions provided by CSR (Center for Space Research, University of Texas at Austin, USA) and CNES/GRGS (Centre National d'Études Spatiales/Groupe de Recherche de Geodesie Spatiale, France) and validated using data from the Tropical Rainfall Measuring Mission (TRMM), water storage changes predicted by the WaterGap Global Hydrological Model (WGHM) and the Global Land Data Assimilation System (GLDAS), and ground-truth (river-gauge) observations.For the challenging Australian case with generally weak hydrological signals, the mascon solutions provide similar results to those from the global solutions, with the advantage of not requiring additional filtering (destriping and smoothing) as, for example, is necessary for the CSR solutions. A further advantage of the mascon solutions is that they offer a higher temporal resolution (i.e., 10 days) compared to approximately monthly CSR solutions. Examining equivalent water volume (EWV) time series for the MDB shows a good cross-correlation (generally > 0.7) among the GRACE solutions when considering the whole basin, although lower (generally 0.6), with all time series appearing to visually follow the general behaviour of the river-gauge data, although the cross-correlations are relatively low (between 0.3 and 0.6).Research Highlights ► Mascon provides equivalent results as global CSR & CNES/GRGS solutions. ► All examined GRACE releases reveal seasonal & tropical north signals. ► GRACE, modelled hydrology & precipitation show similar behaviour Australia wide. ► GRACE solutions generally follow river gauge data

    A Global Gridded Dataset of GRACE Drought Severity Index for 2002–14: Comparison with PDSI and SPEI and a Case Study of the Australia Millennium Drought

    Get PDF
    A new monthly global drought severity index (DSI) dataset developed from satellite-observed time-variable terrestrial water storage changes from the Gravity Recovery and Climate Experiment (GRACE) is presented. The GRACE-DSI record spans from 2002 to 2014 and will be extended with the ongoing GRACE and scheduled GRACE Follow-On missions. The GRACE-DSI captures major global drought events during the past decade and shows overall favorable spatiotemporal agreement with other commonly used drought metrics, including the Palmer drought severity index (PDSI) and the standardized precipitation evapotranspiration index (SPEI). The assets of the GRACE-DSI are 1) that it is based solely on satellite gravimetric observations and thus provides globally consistent drought monitoring, particularly where sparse ground observations (especially precipitation) constrain the use of traditional model-based monitoring methods; 2) that it has a large footprint (~350 km), so it is suitable for assessing regional- and global-scale drought; and 3) that it is sensitive to the overall terrestrial water storage component of the hydrologic cycle and therefore complements existing drought monitoring datasets by providing information about groundwater storage changes, which affect soil moisture recharge and drought recovery. In Australia, it is demonstrated that combining GRACE-DSI with other satellite environmental datasets improves the characterization of the 2000s “Millennium Drought” at shallow surface and subsurface soil layers. Contrasting vegetation greenness response to surface and underground water supply changes between western and eastern Australia is found, which might indicate that these regions have different relative plant rooting depths

    Climate-groundwater dynamics inferred from GRACE and the role of hydraulic memory

    Get PDF
    Groundwater is the largest store of freshwater on Earth after the cryosphere and provides a substantial proportion of the water used for domestic, irrigation and industrial purposes. Knowledge of this essential resource remains incomplete, in part, because of observational challenges of scale and accessibility. Here we examine a 14-year period (2002–2016) of GRACE observations to investigate climate-groundwater dynamics of 14 tropical and sub-tropical aquifers selected from WHYMAP's 37 large aquifer systems of the world. GRACE-derived changes in groundwater storage resolved using GRACE JPL Mascons and the CLM Land Surface Model are related to precipitation time series and regional-scale hydrogeology. We show that aquifers in dryland environments exhibit long-term hydraulic memory through a strong correlation between groundwater storage changes and annual precipitation anomalies integrated over the time series; aquifers in humid environments show short-term memory through strong correlation with monthly precipitation. This classification is consistent with estimates of Groundwater Response Times calculated from the hydrogeological properties of each system, with long (short) hydraulic memory associated with slow (rapid) response times. The results suggest that groundwater systems in dryland environments may be less sensitive to seasonal climate variability but vulnerable to long-term trends from which they will be slow to recover. In contrast, aquifers in humid regions may be more sensitive to seasonal climate disturbances such as ENSO-related drought but may also be relatively quick to recover. Exceptions to this general pattern are traced to human interventions through groundwater abstraction. Hydraulic memory is an important factor in the management of groundwater resources, particularly under climate change

    Estimating the Deep Overturning Transport Variability at 26°N Using Bottom Pressure Recorders

    Get PDF
    The RAPID mooring array at 26°N in the Atlantic has been observing the Atlantic meridional overturning circulation (AMOC) since 2004, with estimates of AMOC strength suggesting that it has declined over the 2004–2016 period. When AMOC transport is estimated, an external transport is added to the observed Ekman, Florida Straits, and baroclinic geostrophic transports to ensure zero net mass transport across the section. This approach was validated using the first year of RAPID data by estimating the external component directly from in situ bottom pressure data. Since bottom pressure recorders commonly show low‐frequency instrument drift, bottom pressure data had to be dedrifted prior to calculating the external component. Here we calculate the external component from 10 years of in situ bottom pressure data and evaluate two choices for dedrifting the records: traditional and adjusted using a Gravity Recovery and Climate Experiment (GRACE) bottom pressure solution. We show that external transport estimated from GRACE‐adjusted, in situ bottom pressure data correlates better with the RAPID compensation transport (r=0.65,p<0.05) than using individually dedrifted bottom pressure recorders, particularly at low frequencies on timescales shorter than 10 years, demonstrating that the low‐frequency variability added from GRACE is consistent with the transport variability at RAPID. We further use the bottom pressure‐derived external transport to evaluate the zonal distribution of the barotropic transport variability and find that the transport variability is concentrated west of the Mid‐Atlantic Ridge rather than uniformly distributed across the basin, as assumed in the RAPID calculation

    Sea-level change over the northern European continental shelf due to atmospheric and oceanic contributions

    Get PDF
    Global mean sea level (GMSL) is a key indicator of climate change as it comprises information on different components of the climate system. However, despite its importance for climate and society, GMSL cannot be used for coastal adaptation policies because regional sea-level variations can significantly depart from the global average. Providing accurate estimates of sea-level rise is therefore one of the most important scientific issues that climate change poses, with a large impact for the human population as it is recognized as the main driver for changes in sea-level extremes, influencing the non-linear interactions between processes acting over different temporal and spatial scales in coastal areas. This thesis addresses different aspects of the sea-level variability over the northern European continental shelf. Paper I uses gridded satellite altimetry data and adopts the jet clusters perspective of the winter-time atmospheric variability over the North Atlantic to reassess the contribution of local winds to the sea-level variability over the northern European continental shelf. By using the jet clusters, Paper I distinguishes itself from the existing literature since the jet clusters provide a physical description of the atmospheric variability in the North Atlantic. Papers II and III focus on the steric and manometric components of the sea-level over the Norwegian section of the northern European continental shelf and on the sea-level observing system in the region. Paper II first evaluates a coastal altimetry dataset, reprocessed with the ALES-retracker, against the Norwegian set of tide gauges. After showing a good agreement between the two, it exploits the coastal satellite altimetry dataset to reassess the steric component of the sea level over the Norwegian shelf: the paper finds that the estimates of the steric component of the sea-level do not depend much on the choice of the tide gauges or satellite altimetry. Paper III evaluates the sea-level observing system along the Norwegian coast by assessing the ability of a satellite gravimetry mission, the Gravity Recovery and Climate Experiment (GRACE), and of a combination of satellite altimetry and hydrography to monitor manometric sea-level variations in the region. It then investigates the open-ocean contribution to the inter-annual manometric sea-level variations along the coast of Norway. It shows that, while commonly considered not reliable in the coastal region, GRACE captures the main features of the manometric sea-level change in the area, which on interannual and longer time scales can be attributed to along-slope winds and open-ocean steric changes. Therefore, GRACE can be used to analyze the manometric sea-level variability, such as in sea-level budget studies, especially in those areas of the coastal ocean where in-situ measurements are sparse. Overall, by focusing on the northern European continental shelf due to its well developed sea-level observing system, this thesis has demonstrated the potential of remote sensing observations in improving our understanding of sea-level variability and change in the coastal ocean.Doktorgradsavhandlin

    Climate–groundwater dynamics inferred from GRACE and the role of hydraulic memory

    Get PDF
    Groundwater is the largest store of freshwater on Earth after the cryosphere and provides a substantial proportion of the water used for domestic, irrigation and industrial purposes. Knowledge of this essential resource remains incomplete, in part, because of observational challenges of scale and accessibility. Here we examine a 14-year period (2002–2016) of Gravity Recovery and Climate Experiment (GRACE) observations to investigate climate–groundwater dynamics of 14 tropical and sub-tropical aquifers selected from WHYMAP's (Worldwide Hydrogeological Mapping and Assessment Programme) 37 large aquifer systems of the world. GRACE-derived changes in groundwater storage resolved using GRACE Jet Propulsion Laboratory (JPL) mascons and the Community Land Model's land surface model are related to precipitation time series and regional-scale hydrogeology. We show that aquifers in dryland environments exhibit long-term hydraulic memory through a strong correlation between groundwater storage changes and annual precipitation anomalies integrated over the time series; aquifers in humid environments show short-term memory through strong correlation with monthly precipitation. This classification is consistent with estimates of groundwater response times calculated from the hydrogeological properties of each system, with long (short) hydraulic memory associated with slow (rapid) response times. The results suggest that groundwater systems in dryland environments may be less sensitive to seasonal climate variability but vulnerable to long-term trends from which they will be slow to recover. In contrast, aquifers in humid regions may be more sensitive to climate disturbances such as drought related to the El Niño–Southern Oscillation but may also be relatively quick to recover. Exceptions to this general pattern are traced to human interventions through groundwater abstraction. Hydraulic memory is an important factor in the management of groundwater resources, particularly under climate change
    corecore